1. Field of the Invention
The present invention is directed to a method for calibration and measurement in a micro-dialysis system, as well as to a micro-dialysis system wherein the method is implemented.
2. Description of the Prior Art
In micro-dialysis a measurement probe is inserted into the measurement area, usually a part of a patient""s body. The measurement probe contains a dialysate that, through a permeable part of the measurement probe, accepts substances diffusing out of the measurement area. The substances can consist of e.g. oxygen ions in blood. The dialysate is subsequently carried to a sensor for measuring the diffusing substance. This provides a measure of a specific parameter in the measurement area, e.g. the blood gas value for oxygen.
One problem in this procedure is to ensure that the sensor measures a relatively correct value. If the sensor does not measure a direct value, it should be possible to at least calculate the correct value.
In principle, two factors have a predominant impact on sensor accuracy: the measurement probe""s recovery ability and the sensor""s signal drift.
Recovery ability R is defined as   R  =            C      dialysate              C              measurement        ⁢                  xe2x80x83                ⁢        area            
where C stands for the concentration of the diffusing substance (in the dialysate and measurement area respectively).
Recovery ability depends, in turn, on factors such as temperature, the material the measurement probe is made of, the flow of dialysate and convection around the measurement probe. Several of these factors vary with time, and the system must operate very slowly, or recovery must be monitored in some way, to assure 100% recovery between each measurement. If recovery is monitored, a calculation can be made to compensate for measurements made before 100% recovery has taken place.
An object of the present invention is to provide a method for micro-dialysis which solves the aforementioned problems.
Another object of the invention is to provide a micro-dialysis system capable of performing measurements more rapidly and accurately than known micro-dialysis systems.
The above object is achieved in accordance with the principles of the present invention in a method for calibration and measurement of a micro-dialysis system, and a micro-dialysis system wherein the method is implemented, employing a sensor wherein calibration and measurement are performed with two dialysates respectively containing different analytes.
A more effective analysis method can be employed when two dialysates, with differing analyte contents with respect to the parameter to be identified, are used. The two dialysates are employed so that calibration and measurement are performed with one dialysate at a time. The first dialysate is used at appropriate intervals, preferably for every second calibration and measurement. The second dialysate is used for the remaining calibrations and measurements.
100% recovery is then no longer necessary, since two-paint calibration of the sensor becomes possible. This is accomplished by utilization of the three most recent sensor readings for determining a measurement value, i.e., two calibrations and the actual measurement.
The micro-dialysis system is devised to enable two dialysates to be used alternately in a suitable fashion for performing the method described above.